Energy Secretary Chris Huhne will learn on Friday whether he is to be prosecuted over claims he lied to avoid a speeding ban, in a decision that could derail his political career and force a cabinet reshuffle.
If charged, Huhne, a Liberal Democrat who has promoted a strategy of green and renewable power, will face intense pressure to step down from the government.
Two ministers have already quit because of scandals since the Conservative-led coalition took power in May 2010.
England's top prosecutor, Keir Starmer, will announce on Friday morning whether he plans to charge Huhne or his estranged wife, Vicky Pryce, over the claims.
Huhne has denied any wrongdoing over allegations he asked Pryce to take responsibility for a speeding offence in 2003 that would otherwise have cost him his driving licence. Pryce, a former government economist, has remained silent over the alleged offence, which took place in Essex, east of London.
The couple split in 2010.
Huhne is a wealthy former journalist who was a member of the European Parliament from 1999 to 2005. Elected to parliament in 2005, he stood for his party leadership the following year but lost to Nick Clegg, now deputy prime minister.
At last December's United Nations climate change talks in Durban, Huhne was credited with helping hammer out a landmark international agreement on the cutting of greenhouse gas emissions.
Last month Clegg told the BBC that criminal charges against Huhne would be a very serious issue, while noting that the energy minister had denied any wrongdoing to him personally.
If Huhne is forced to resign, Lib Dem Employment Minister Ed Davey would be a likely choice to replace him, party insiders say. The Lib Dems are ensured of five senior cabinet positions to maintain the coalition balance.
The two ministers who have stepped down are Lib Dem David Laws, a treasury minister, three weeks after the government coalition was formed, and Conservative Defence Secretary Liam Fox in October.
(Additional reporting by Michael Holden and Oleg Vukmanovic; Editing by Alessandra Rizzo)